Anyway, the constant c would still be of great importance (and would still be measurable) even if there were no particles that moved that fast, assuming the laws of physics were still Lorentz-symmetric. For example, clocks moving at some v slower than light would still slow down by the factor of [tex]\sqrt{1 - v^2/c^2}[/tex].

im not sure, but this is what i think:
Hypothesis of why the speed of light is the speed of light:
In the time of Big Bang when the first atoms were created, the matter and anti-matter destroyed eachother and made the photons. When they eliminated each other, photons were sent out, in the exact speed it was accelerated in. And as Newton’s laws says: “Nothing can be stoppep without friction”. And, as we all know, photons are massless particles and have no effect on gravity, and that’s why they never stop and always go in the same speed. :D PEACE!